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doc: Add note about I/O timing information in EXPLAIN and pg_stat_database.
Explain that the followings are tracked only when track_io_timing GUC is enabled. - blk_read_time and blk_write_time in pg_stat_database - time spent reading and writing data file blocks in EXPLAIN output with BUFFERS option Whther track_io_timing is enabled affects also blk_read_time and blk_write_time in pg_stat_statements, but which was already documented. Author: Atsushi Torikoshi Reviewed-by: Fujii Masao Discussion: https://postgr.es/m/CACZ0uYHo_NwbxpLH76OGF-O=13tkR0ZM0zeyGEhZ+JEXZVRyCA@mail.gmail.com
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doc/src/sgml/monitoring.sgml

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@@ -3390,7 +3390,8 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
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</para>
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<para>
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Time spent reading data file blocks by backends in this database,
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in milliseconds
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in milliseconds (if <xref linkend="guc-track-io-timing"/> is enabled,
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otherwise zero)
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</para></entry>
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</row>
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@@ -3400,7 +3401,8 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
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</para>
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<para>
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Time spent writing data file blocks by backends in this database,
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in milliseconds
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in milliseconds (if <xref linkend="guc-track-io-timing"/> is enabled,
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otherwise zero)
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</para></entry>
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</row>
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doc/src/sgml/ref/explain.sgml

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@@ -171,8 +171,9 @@ ROLLBACK;
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<para>
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Include information on buffer usage. Specifically, include the number of
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shared blocks hit, read, dirtied, and written, the number of local blocks
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hit, read, dirtied, and written, and the number of temp blocks read and
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written.
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hit, read, dirtied, and written, the number of temp blocks read and
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written, and the time spent reading and writing data file blocks
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(in milliseconds) if <xref linkend="guc-track-io-timing"/> is enabled.
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A <emphasis>hit</emphasis> means that a read was avoided because the block was
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found already in cache when needed.
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Shared blocks contain data from regular tables and indexes;

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